Well I would suggest you wait until next spring now to make the feed, or 'tea', but he method is very simple:

Cut up the fresh plants into small pieces and soak them in water (use 1 kg/2 lb of plants per 10 litres/15 pints of water) and leave it to steep for between one and four weeks – use a wooden or stoneware container, or even plastic if necessary, never use a metal container. Stir the liquid from time to time to help it ferment; then filter out the plant pulp. Use the resulting liquid fertilizer diluted in the proportion of 1:10–20, as appropriate. — from In Tune with the Moon

doh! If I had brains................... Never occured to me to strain it. Thanks for the instructions, now just one more question, how do I weigh out 2lb of leaves when at the allotment? On second thoughts rather than worry about transporting my kitchen or bathroom scales I'll just stuff a bucket etc but remember to strain it as that seems to be where I've been going wrong!

Take a look at the weight of a cabbage or or some chard leaves - this should give you an idea of how much comfrey to put in your bucket :-)